Hearing aids are delicate electro-acoustical devices that are intended to perform flawlessly in the harsh environment of the human ear. Hearing aid manufacturers use hard durable earmold materials to protect the delicate electronics and transducers from adverse conditions in and out of the ear. Such durable materials are intended to survive a greater than four year usage life. The earmold can be used not only to protect the internal components of the hearing aid, but also to form an interface between the electronics and the ear canal. This interface must be created to prevent acoustical feedback, to retain the electronics in the ear and prevent irritation or fatigue of the ear canal. A design conflict for the earmold can then exist between the need to have a soft, pliant, nonirritating and comfortable material, and the need to have a durable material to protect the internal electronic components which can last more than four years.
A disposable hearing aid is one designed to have a useful life of much less than the traditional four years. The useful life of a disposable hearing aid can also be governed by the life of the battery. If the battery can be replaced, the hearing aid is no longer disposable and the advantages of the disposable aid are lost. However, a number of other variants on the disposable aid are possible. These variants look to maintain the advantages of disposability while making the system more convenient, comfortable, longer lasting or providing better performance.
“One-size-fits-all” type hearing aids generally lack a flexibility of their tips. Such a lack of flexibility prevents deep penetration into the bony region of an ear canal by the hearing aid. Such hearing aids are also unable to create an acoustic seal in the ear canal. Without such penetration or such a seal in the bony region, the hearing aid can create an occlusion effect in the wearer. Also, hearing aids lacking a proper acoustic seal are prone to feedback.
Feedback is a nagging, unsolved problem for hearing aid manufacturers and dispensers. Feedback in a hearing is present in several forms: electrical, acoustical and mechanical. While electrical feedback is more easily controllable with additional circuitry, the latter two forms of feedback in hearing aids remain significant problems that often reduce the effectiveness of hearing aid fittings. Acoustical or mechanical feedback signals from the hearing aid receiver may be transduced by the hearing aid microphone, amplified in the circuitry and output from the receiver again but with higher gain, creating a feedback loop, and potentially oscillation. A criteria by Nyquist states that a system will oscillate if the gain of the open loop transfer function at a particular frequency is greater than unity and its phase shift of the open loop transfer function is a multiple of 360 degrees. Acoustical feedback occurs through the air inside of the hearing aid housing as well as on the outside of the hearing aid while it is being worn. Mechanical feedback occurs because of the receiver vibrating the hearing aid housing. Hearing aid designers have used many techniques to prevent the creation of such feedback loops. Included are rubber isolation mounts for the microphone and receiver, stiff tubes on the receiver output port and electronically filtering the hearing aid output to suppress energy at the most likely to be troublesome feedback frequencies. Unfortunately, hearing aids frequently have feedback oscillation frequencies in the very range at which the wearer requires amplification most, such as in the frequency range between 1500 and 5000 Hz.